1. Field of the Invention
The present invention relates to a sheet reversing unit for reversing a sheet between printing units for two-side printing.
2. Description of the Prior Art
A printing press which can be selectively used for both single-side printing and two-side printing has been commercially available to satisfy various needs in printing. A printing press of this type has a sheet reversing unit for reversing a sheet whose one surface has already been printed and then conveying it to the next printing unit for two-side printing.
The two-side printing operation of the printing press having this sheet reversing unit will be described with reference to FIG. 3. Two printing units 1 and 2 of the printing press have a rubber blanket cylinder 3, each of which is in contact with a corresponding plate cylinder (not shown), and impression cylinders 4 and 5, each of which is in contact with the corresponding rubber blanket cylinder 3 and each of which has a diameter twice that of the blanket cylinder 3. Gripper devices 6 and 7 (to be referred to simply as grippers 6 and 7 hereinafter) each consist of a gripper and a gripper pad which are arranged in notches that are formed at a position that equally divides the outer peripheries of the impression cylinders 4 and 5. A reversing cylinder 8 is disposed between the two impression cylinders 4 and 5 so that their peripheral surfaces are brought into contact with each other. A reversing device or unit 11 consisting of a reversing gripper 9 and a suction lever 10 is arranged in a notch formed at a position that equally divides the outer periphery of the reversing cylinder 8. Reference numeral 12 denotes a feeder board for stopping a sheet 13 fed from the sheet feeder and registering the sheet end. Reference numeral 14 denotes a swing device for gripping the sheet 13 on the feeder board 12 to be re-gripped by the gripper 6 of the impression cylinder 4.
With the above arrangement, the sheet 13 gripped by the gripper 6 of the impression cylinder 4 is conveyed while being wound around the peripheral surface of the impression cylinder 4 upon rotation of the impression cylinder 4 in the direction indicated by the arrow in FIG. 3. When the sheet 13 passes through the cylinders 3 and 4, an image transferred from the plate cylinder to the rubber blanket cylinder 3 is transferred to a front surface of the sheet 13, thus performing surface printing. When the impression cylinder 4 is further rotated, and the gripper 6 gripping the sheet 13 has passed the contact point between the two cylinders 4 and 8 and the sheet end of the sheet 13 reaches the contact point between the cylinders 4 and 8, as shown in FIG. 3(b), the suction lever 10 draws the sheet end which causes the reversing gripper 9 to grip it, and the gripper 6 releases the sheet 13. FIG. 3(c) illustrates a state wherein the respective cylinders have been rotated through about 30.degree. from the re-gripping operation. The sheet 13 gripped by the reversing gripper 9 is about to be wound around the reversing cylinder 8 as it was wound around the impression cylinder 4 so that its non-printing surface faces the peripheral surface of the reversing cylinder 8. Reference numerals 13A and 13B denote proceeding sheets; and 13C and 13D, following sheets. The gripped end of the sheet 13 wound around the reversing cylinder 8 reaches the contact point between the cylinders 8 and 5 upon rotation of the respective cylinders like the sheet 13B, and the sheet 13 is re-gripped from the reversing gripper 9 to the gripper 7. When the respective cylinders are further rotated, the sheet 13 is reversed so that the printed surface faces the peripheral surface of the impression cylinder 5. The sheet 13 is then wound around the impression cylinder 5. Thereafter, in the printing unit 2, the printing operation is performed as in the printing unit 1. In this case, since the sheet 13 is reversed, an image is printed on its back surface.
The two-side printing operation has been described schematically. The arrangement and operation of the reversing unit 11 will be described in more detail with reference to FIGS. 4 and 5. FIG. 4 is an enlarged side view of the reversing unit which has passed the contact point with the impression cylinder 5 and is rotated toward the contact point with the impression cylinder 4, and FIG. 5 is an enlarged side view of the reversing unit at the contact point with the impression cylinder 4. In FIGS. 4 and 5, a gripper shaft 15 and a hollow suction lever shaft 16 are axially suspended in the notch of the outer peripheral portion of the reversing cylinder 8 and parallel to a cylinder shaft. These shafts 15 and 16 are alternately oscillated in the normal and reverse directions through a predetermined angle at a predetermined timing through the mesh between segment gears which are driven by a cam mechanism arranged on a frame (not shown) and pinions on the shafts 15 and 16. The reversing gripper 9 consists of a gripper holder 17 fixed to the gripper shaft 15 and a pivotal gripper pad holder 18. A gripper 20 for gripping the sheet 13 between itself and a gripper pad 19 fixed to the distal end portion of the gripper pad holder 18 is pivotally mounted on the distal end portion of the gripper pad holder 18. Reference numeral 21 denotes a gripper spring for biasing the gripper pad holder 18 in the closing direction of the gripper 19; and 22, a gripper spring for applying a gripping pressure to the gripper 19. The reversing gripper 9 with the above arrangement is pivoted between positions shown in FIGS. 4 and 5 together with the gripper shaft 15 which is reciprocated by the cam mechanism. A roller lever which is swung by the cam mechanism on the frame side (not shown) is fixed to one end of the gripper shaft 15, so as to open and close the gripper 20 at the position shown in FIG. 5. Additionally, the suction lever 10 consists of an L-shaped lever 23 which is fixed to the suction lever shaft 16 at a phase different from that of the reversing gripper 9 in the axial direction, and a suction port 24 mounted on one free end portion. The suction port 24 is connected to a suction air source through a hollow portion of the suction lever shaft 16, a rotary valve, and the like. The suction lever 10 is pivoted, between the positions shown in FIGS. 4 and 5, together with the suction lever shaft 16 which is reciprocated by the cam mechanism. Note that a stopper 25 projecting from the lever 23 is formed to have the same phase as that of the gripper pad holder 18 in the axial direction.
With the above arrangement, when the reversing unit 11 is located on the upper side of the reversing cylinder 8, the reversing gripper 9 is stopped in position while a stopper 26 abuts against a stopper 27 of the reversing cylinder 8, as shown in FIG. 4. The suction lever 10 is stopped in position while the stopper 25 abuts against the gripper pad holder 18. When the respective cylinders are rotated and the reversing unit 11 reaches the position shown in FIG. 5, only the suction lever 10 is pivoted counterclockwise by the cam mechanism to the position shown in FIG. 5, and then the reversing gripper 9 is pivoted clockwise by another cam mechanism to the position shown in FIG. 5. Thus, since the stopper 25 is separated from the tail portion of the gripper pad holder 18, the suction lever 10 is again returned to the position shown in FIG. 4, causing the sheet end to be drawn by the suction port 24, and is then pivoted counterclockwise to provide the state shown in FIG. 5. At this time, the gripper shaft 15 is oscillated through a predetermined angle by another cam for opening and closing the gripper, and only the gripper holder 17 is opened and closed, thereby gripping the sheet end between the gripper 20 and the gripper pad 19. When the respective cylinders are further rotated, the gripper opening and closing cam is disabled, and a reversing cam pivots the gripper shaft 15, so that the reversing gripper 9 gripping the sheet end and the suction lever 10 are returned to the positions shown in FIG. 4. In this case, since the suction lever 10 returns to the position shown in FIG. 4 slightly after the reversing gripper 9 does, the stopper 25 does not interfere with the tail portion of the gripper pad holder 18. When the reversing gripper 9 and the suction lever 10 are returned to the positions shown in FIG. 4, a cam follower of the cam mechanism for driving them is moved along a downwardly inclined surface from a large-diameter portion to a small-diameter portion of the cam surface, and the cam follower is urged against the cam surface by the biasing force of a spring member.
However, in the conventional sheet reversing unit as described above, when the reversing gripper 9 is returned from the position shown in FIG. 5 to the position shown in FIG. 4, the cam follower is moved along the downwardly inclined surface while being biased by the spring. When the printing press is operated at high speed, the biasing force of the spring cannot follow the rotation of the cam, and the cam follower floats from the cam surface, resulting in so-called cam skip. This may delay a pivot timing of the reversing gripper. As a result, when the suction lever 10 is pivoted after the reversing gripper 9 catches up with the cam surface, the stopper 25 interferes with the tail portion of the gripper pad holder 18, as shown in FIG. 6, so that the tail portion or its surrounding parts may be damaged. In particular, this phenomenon may often occur when the printing mode is switched from the single-side printing mode to the two-side printing mode for long periods and when high-speed printing is performed, resulting in poor reliability.